Typical precision patterned fill processes, for example, in semiconductor manufacturing (e.g., integrated circuits, chip technology, and chip packaging), provide filling of features (cavities or trenches created, for example, by etching) on a wafer or semiconductor chip. The features may be filled with substances including pastes, inks, liquid metals (such as solder) and solvents. These materials may be at sub-ambient, ambient, or high temperatures such as molten solders. Further, the features may be such features and cavities required in manufacturing of a product, including small features, for example, 5-200 μm wide and/or deep.
One problem associated with current patterned fill processes is that pressure alone is often not sufficient to inject materials into the features. Moreover, for example, through holes of high aspect ratio, i.e., comparatively large height and diameter, or height and width, can be difficult to fill. Furthermore, blind holes are often very difficult to fill since entrapped gas backpressure can prevent complete filling of the holes.
Typically, there are problems filling holes or mold features using cavity filling processes due to the presence of ambient atmosphere gas in the features. The gas must be completely displaced by the filling material or gas pockets compromise the filled feature and/or can cause a break in a seal around the feature. The problem is accentuated during high speed fills where the feature or cavity has minimal time to bleed out the entrapped gas while the fill material enters the cavity. Thus, the displacement process often is incomplete in the time desired for filling features, and results in partially filled or in extreme cases empty cavities which become defects in the process. For some operations, no defects, such as partial or unfilled cavities of features are allowable. Entrapped gases in the features may result in a partially filled cavity. A partially filled feature or cavity may result in seal degradation around the feature, especially over extended periods of time at high temperatures, e.g., over 200° degrees Celsius.
Another problem with current feature filling processes is that current attempt to seal the feature are inadequate to maintain the seal around the feature, as the surface area may be rough. The roughness may be caused by current sealing methods which may drag the fill substance, such as solder, from the cavities leaving streaks on the surface area of the device, e.g., wafer.
Referring to FIG. 1, a known fill head assembly 10 for dispensing molten solder into a mold plate uses a fill head 20. The fill head assembly 10 further includes a solder reservoir 12 being partially filled with solder 14. A body portion 30 of the assembly 10 includes two heater 32 for heating the solder 14 in the reservoir 12. A passageway 18 provides an inlet for the solder and is pressurized with a downward pressure 19. A solder fill region 16 or solder outlet in the solder fill head 20 provides egress for the solder 14. Two seals 24 are positioned on opposite sides of the solder fill region 16. A mold plate 40 (for example, a glass mold plate) includes cavities 44. Using the assembly 10, the body portion 30 is heated to above the melting point of solder using built-in cartridge heaters 32. For example, tin or tin alloy solders melt at approximately 230 degrees C., therefore in this case the body portion is heated to around 250 degrees C. The molten solder 14 is held in the sealed reservoir 12. The fill head (alternatively FH or solder fill head) assembly 10 rests on the mold plate 40 and a nominal load or downward force is applied (typically 2.5 lbs/linear inch of seal). A seal at the solder in solder outlet 16 prevents the solder 14 from leaking out the bottom of the fill head assembly 10. The solder reservoir 12 is pressurized, usually to a pressure of between 0 and 20 psi, to ensure that solder enters the mold plate 40 cavities 44 during the mold fill process. The small cavities 44 in the mold plate 40 are filled by moving the mold plate 40 underneath the solder fill head 20, typically at a speed of between 0.1 to 10 mm/sec. Air is purged from the mold plate cavities as the solder enters the cavities. The air escapes between the seal 24 and a top surface 42 of the mold plate 40. This process continues until all mold cavities 44 are filled. The mold plate 40 is moved in the direction 41. The mold plate 40 with the filled cavities 45 is then removed and passed to the next tool where the solder is transferred from the mold to the pads of a silicon wafer.
Shortcomings with current methods of solder fill described above include the solder must exert pressure on the air in the cavities to force the air from the cavities. This pressure may cause the solder to leak from the seal in the outlet 16, particularly if there are variations in the seal or variations in the flatness of the mold plate. Another problem is that for air from the cavities to escape across the seal it is helpful if the seal is roughened, textured, or scratched to provide small channels to enable the air to more easily escape between the seal and the top surface of the mold. However, this approach results in increased wear over time, for instance wearing away the channels or scratches, resulting in the same problem as the channels where to prevent, i.e., difficulty in purging the air from the cavities. An additional problem with current approaches is that even with the textured or channeled seal discussed above, pressure alone may not be sufficient to eject air from the cavities, thereby unwanted air remains in the cavities resulting in the undesirable condition of partially filled cavities, i.e., cavities partially filled with solder.
Other known fill head assemblies include a solder dispensing region, a vacuum region, a flat seal, and channels or slots that enable communication between the vacuum region and the solder region. The vacuum region is intended to remove the air from the mold plate cavities prior to fill. However, several deficiencies of known designs include difficulty in maintaining desired contact between the solder fill head assembly and the mold plate by using a flat seal. For example, even if a compliant seal material is used, irregularities in the mold plat e surface and alignment errors between the fill head assembly and the mold plate result in solder leaking across the seal. It is also difficult to maintain a vacuum in the mold plate cavities prior to solder fill due to air leaking into the vacuum region. Another problem with current designs is that as the seal wears, small slots between the vacuum region and the solder region tend to disappear, thus making it difficult to maintain a good vacuum in the mold plate cavities prior to solder fill. Another problem with current designs is that a flat seal does not provide adequate wiping as it moves across the mold plate, and therefore tends to leave streaks of solder on the mold surface.
It would therefore be desirable to provide a localized vacuum environment to remove ambient gas and encourage backfilling of a material used to fill features during manufacturing. It would also be desirable to provide an apparatus and method for filling features with material at high speed, without material overfill, bridging, or streaking. Further, there is a need for a reliable mold filling process which ensures that each cavity or feature is accurately filled by a fill head device.